Tens of thousands of people who were evacuated from near the Fukushima Daiichi nuclear plant risk arrest if they return home, after the government declared the area a no-entry zone due to high radiation levels.

Under the order, which goes into effect at midnight local time, people living within a 12-mile (20km) radius of the atomic plant will be given up to two hours to enter the area to collect belongings.

The move came amid concern over the long-term health risks posed by high levels of accumulated radiation, despite signs of progress in bringing the stricken facility under control.

The 245 workers battling to stabilise Fukushima have fallen ill due to the harsh conditions inside the plant, experts warned.

Some are suffering from insomnia, dehydration and high blood pressure, and risk developing depression or heart trouble, Takeshi Tanigawa, chairman of the public health department at Ehime University's medical school, told Associated Press.

The government has also extended the evacuation zone to several locations outside the 12-mile zone, including areas in which as many as 130,000 people had initially been asked to leave voluntarily or stay indoors. Residents in those areas will be given a month to evacuate.

The government's chief spokesman, Yukio Edano, urged people living inside the no-entry zone to abide by the order for the sake of their health.

"The plant is not stable," he told reporters. "We have been asking residents not to enter the area as there is a huge risk to their safety. We beg the understanding of residents."

The no-entry edict prompted residents to rush back into the zone to grab as many belongings as they could before the order went into effect. Some wore white protective suits, facemasks or wet-weather gear they hoped would protect themselves from radiation.

A stream of cars, windows closed, later left the deserted neighbourhoods crammed with clothes and valuables.

"This is our last chance, but we aren't going to stay long. We are just getting what we need and getting out," Kiyoshi Kitajima, an X-ray technician who briefly returned to the hospital where he worked, told Associated Press.

The no-go order angered other evacuees who have been unable to make even a short visit home. "I initially thought we would be able to return within a few days, so I brought nothing with me except a bank card," said Kazuko Suzuki, who fled her home in Futaba with her teenage son and daughter.

"I really want to go back. I want to check if our house is still there. My patience has run out. I just want to go home."

Under the order, people who enter the zone without permission face fines of up to 100,000 yen (£740) and possible arrest.

Almost all of the 80,000 people living in the 12-mile zone have been evacuated, but some have refused to abandon their livestock or move from their homes into evacuation centres.

Police said about 60 families defied the evacuation order imposed when the crisis began, but some had been persuaded to leave.

The contaminated bodies of as many as 1,000 people who died in the 11 March earthquake and tsunami have yet to be recovered from the area.

Edano said that over the next one to two months, one resident per household would be permitted to return home on chartered buses to collect belongings.

They will be required to undergo radiation screening on their way out.

Those living within three kilometres of the plant and other areas where very high levels of radiation have been detected will not be allowed to return, even for a short period, the Kyodo news agency reported.

"We realise this is extremely inconvenient for residents, but we urge you to be patient," Edano said.

The prime minister, Naoto Kan, flew to Fukushima prefecture on Thursday to explain the order to officials and evacuees. Kan, whose approval rating has fallen over his handling of the nuclear crisis, said the government would enforce the no-go zone order.

He asked the prefecture's governor, Yuhei Sato, to help win the understanding of local communities, according to Kyodo.

Tens of thousands of people affected by the nuclear crisis, many of whom fled with nothing but the clothes they were wearing, face many more months living in evacuation centres.

The plant's operator, Tokyo Electric Power [Tepco], said last week it would take between six and nine months to bring down radiation levels and make the facility safe, a timeline some experts have described as optimistic.

Edano conceded that Tepco had not been adequately prepared to cope with the disaster. "Leaving aside the question of whether the accident could have been predicted, it is clear that there was insufficient preparation.

"We urge all nuclear plant operators to immediately take every possible precaution in light of the Fukushima disaster."

A Tepco official on Wednesday admitted that fuel inside the plant's No 1 reactor could be melting. ''I can't say with absolute certainty that [the fuel] has not melted," Junichi Matsumoto said, adding that the firm had been unable to confirm the condition of the reactor's core.

Tanigawa, who has interviewed 80 workers, said fatigue, not radiation, posed the greatest immediate threat to their health. "Working conditions at the plant are harsh," he said. "I am afraid that if this continues we will see a growing risk of health problems."

The workers, along with firefighters and police officers, get little rest and are unable to take baths or eat fresh food, Tanigawa said, adding that they were under constant threat from radiation. In some parts of the plant, contamination levels are so high that only robots have been allowed to enter.

Tanigawa said the conditions contravene basic rights guaranteed by the constitution. When they are not working, the mostly middle-aged men sleep on the floor or a gymnasium at the nearby Fukushima Daini plant.

The workers, who put in four straight days, followed by two off, are exhibiting signs of acute stress brought on by fear of radiation and pleas from family members to leave their jobs. Some also lost homes and relatives in the tsunami.

An anonymous worker who had recently been inside the No 2 reactor's turbine building likened the site to a battlefield. "I work at the plant because I want to save my hometown," he told Asahi TV. "We have worked at the plant all this time. Who else would take on the job if we stopped?"

Tanigawa accused Tepco and the government of failing to support the workers. "They have been struggling for a month without rest," he said. "Tepco and the government don't think about them. They must do their jobs well, but they don't have any support."

In response to the criticisms, Tepco said it would strive to improve conditions at the site. "We think we have worked to improve food, sleeping hours and days off, so the conditions are getting better," it said in a statement. "We would like to make further improvements, taking Dr Tanigawa's views into account."

Michihiro Kono watched as last month's tsunami destroyed his home and the soy sauce business handed down through generations of his family. He describes what happened on 11 March and explains why rebuilding local industry is vital for the area's recovery